Water-Cooled (sub)-Neptunes Get Better Gas Mileage
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866909579823022080 |
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| author | Yoshida, Tatsuya Gaidos, Eric |
| author_facet | Yoshida, Tatsuya Gaidos, Eric |
| contents | The demographics of sub-Jovian planets around low-mass stars is dominated by populations of ``sub-Neptunes" and ``super-Earths", distinguished by the presence or absence of envelopes of low-molecular weight volatiles, i.e., H2, He, and H2O. The current paradigm is that sub-Neptunes on close-in orbits evolve into super-Earths via atmospheric escape driven by high-energy stellar irradiation. We use an integrated hydrodynamic-radiation-chemical network model of outflow to demonstrate that this escape is modulated by the abundance of H2O, an efficient infrared coolant. Increasing H2O/H2 at the base of the flow induces an order-of-magnitude decline in escape rate, with definitive consequences for retention of envelopes over Gyr. We show that saturation limits on H2O in the upper atmospheres of temperate sub-Neptunes could explain the paradoxical observations that these objects disappear more rapidly than their counterparts closer to their host stars. We also propose that the scarcity of sub-Neptunes around very low mass stars could be related to the water-poor chemistry of their antecedent protoplanetary disks. Observations of atmospheric H2O by JWST as well as searches for atmospheric escape from younger planets using H and He lines could test these predictions. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2503_23020 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Water-Cooled (sub)-Neptunes Get Better Gas Mileage Yoshida, Tatsuya Gaidos, Eric Earth and Planetary Astrophysics The demographics of sub-Jovian planets around low-mass stars is dominated by populations of ``sub-Neptunes" and ``super-Earths", distinguished by the presence or absence of envelopes of low-molecular weight volatiles, i.e., H2, He, and H2O. The current paradigm is that sub-Neptunes on close-in orbits evolve into super-Earths via atmospheric escape driven by high-energy stellar irradiation. We use an integrated hydrodynamic-radiation-chemical network model of outflow to demonstrate that this escape is modulated by the abundance of H2O, an efficient infrared coolant. Increasing H2O/H2 at the base of the flow induces an order-of-magnitude decline in escape rate, with definitive consequences for retention of envelopes over Gyr. We show that saturation limits on H2O in the upper atmospheres of temperate sub-Neptunes could explain the paradoxical observations that these objects disappear more rapidly than their counterparts closer to their host stars. We also propose that the scarcity of sub-Neptunes around very low mass stars could be related to the water-poor chemistry of their antecedent protoplanetary disks. Observations of atmospheric H2O by JWST as well as searches for atmospheric escape from younger planets using H and He lines could test these predictions. |
| title | Water-Cooled (sub)-Neptunes Get Better Gas Mileage |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2503.23020 |